This invention relates to video signal processing systems, especially though not exclusively for producing so called production effects in television.
In our co-pending Patent Application No. 8306789, which corresponds to U.S. application Ser. No. 475,790, there is described a video signal processing system in which production effects are produced by writing input video signals, received in television raster format, into storage locations of a frame store so chosen that, when the signals are subsequently read from the storage locations in television raster format, the signals are re-arranged in the raster to alter the image in shape, size or position, or in some other way. A change in size may for example produce the effect of a zoom in or out; and the zoom may be accompanied by some other effect such as rotation or scroll. In order to achieve the selection of storage locations for the input signals, a shape store is provided containing, at positions which correspond to respective pixel positions in the television raster, address signals identifying storage positions in the frame store at which the input video signals should be written in order to achieve the desired effect. A set of address signals describing a desired shape or other characteristic of an image is called an address map. Normally the address map is only coarsely subdivided, comprising, say, an address for each eighth pixel, in each eighth line of a frame. Sequences of address maps are provided to produce changing effects, successive maps in a sequence describing, say, every fourth field. The addresses are thus distributed on a coarse grid, and interpolating means are provided to produce addresses for the intervening pixels and also for the intervening fields. This allows addresses to be read at a rate less than "real time", but to be used after interpolation for writing the input video signals on the frame store in real time.
The stored maps are entered in the shape store from a computer having a keyboard or other control means by which desired production effects can be entered into the computer. The computer is arranged to generate the addresses for desired maps under software control.
Each storage location in the frame store in which the rearranged video signals are written will correspond to a pixel in the output signal television raster. However, in general, an address provided by the computer and the interpolating means will not coincide with a storage location in the frame store but will lie within a rectangular area defined by four storage locations. Therefore on writing an input video signal (relating to a pixel in the input signal raster) in the frame store, it is usually necesssary to distribute the signal among four storage locations by interpolation (this interpolation being different from the address interpolation previously referred to). This means that up to four storage locations have to be accessed for each address. As a corollary, each storage location has to be accessed a plurality of times in response to different addresses.
The system described in the aforesaid patent application represents a powerful means of producing video effects in real time. A difficulty may however be encountered when the effect produced comprises or includes a zoom in.
As the zoom progresses, the addresses provided by the computer and the interpolating means move further apart so that the separation of the addresses for adjacent input pixels may exceed the separation of storage locations in the frame store. When this occurs some storage location in the frame store will not receive any contributions from the input pixels or may receive only a partial contribution, and the image when produced appears to break up. This is illustrated in FIG. 1 of the accompanying drawing in which a number of storage locations in the frame store are represented by small circles. The addresses of a few of the location are represented as x.sub.n, y.sub.n ; x.sub.n+1, y.sub.n ; x.sub.n, y.sub.n+1 ; and x.sub.n+1, y.sub.n+1 respectively. The crosses on the other hand represent a few addresses provided by the computer and the interpolating means for pixels of the input video signals. The addresses denoted in the drawing as x.sub.k, y.sub.k ; x.sub.k+1, y.sub.k and x.sub.k+2, y.sub.k comprise the addresses, or three successive pixels, a, b and c in one line l.sub.n of the input signals, whilst the addresses x.sub.k, y.sub.k+1 ; x.sub.k+1, y.sub.k+1 and x.sub.k+2, y.sub.k+1 comprises the addresses for three successive pixels, d, e and f in the next line l.sub.m+1 in the same field of the input signals.
As each input pixel arises in the input video signals it is written at the address provided for it by the computer and the interpolating means, such writing being achieved, as may be illustrated by the pixel a, by distributing the pixel to the four neighbouring storage locations in the frame store, namely in the case pixel a, the locations at x.sub.n, y.sub.n ; x.sub.n+1, y.sub.n ; x.sub.n, y.sub.n+1 and x.sub.n+1, y.sub.n+1 the distribution being effected in proportions related to the overlap of the pixel a on notional pixels at the neighbouring storage locations. The output video signals, incorporating the required production effect, are derived by reading the signals stored in the storage locations in television raster format. This is the case illustrated in FIG. 1, the output signals are derived by reading the signals from successive location in line l.sub.m and then in line l.sub.m+1, and similarly for other lines.
Consideration of FIG. 1 will reveal that it represents the production of a zoom in, the zoom factor as between the output and input signals being less than 1:2. The figure also shows that many of the storage locations in the frame store will receive no contributions from the input pixels, such as the locations on line l.sub.o and the column C.sub.o. This leads to apparent break up of the image, when the output signals are reproduced.
This undesirable phenomenon is not confined to zoom in effects and may occur locally in a picture when a shape change leads to magnification of part of the image. As already indicated, it may also occur whilst the image is being rotated or otherwise shifted in position or changed in shape.